The present invention relates to coupling structures which convert electrical signals from one transmission medium to another, and more particularly to coupling structures which convert electrical signals from planar transmission lines to waveguides.
As is known in the art, electrical signals may be conveyed by a number of transmission mediums, including electrical traces on circuit boards (e.g., transmission lines), waveguides, and free-space. In many applications, one or more electrical signals are converted from one transmission medium to another. Structures which convert signals from one medium to another are called coupling structures. Such structures for coupling from circuit board traces to waveguides have become increasingly popular due to their growing applications in the area of low-cost packages for monolithic microwave integrated circuits (MMICs), particularly for MMICs which process signals in the millimeter-wave frequency bands.
In most of the prior art circuit-board to waveguide coupling structures, a metal cavity or a metal short on a different plane is used to achieve impedance matching to the waveguide and to avoid back scattering from the waveguide. In some cases, the distance of the back metal short from the planar circuit sets the frequency of operation, which is not always desirable. Instead of using a back metal short, other prior art structures use a quarter-wavelength long dielectric slab inserted into the waveguide to achieve better impedance matching. Such a dielectric slab can have a metal patch disposed on one of its surfaces, or it may be left blank. For these dielectric-slab embodiments, package costs become quite high due to the difficulties in the mechanical fitting and alignment of the dielectric slab inside the waveguide wall.
In view of the prior art, there is a need for a planar transmission line to waveguide coupling structure which does not impose constraints on the frequency of operation, and which is relatively inexpensive to manufacture. The present invention is directed to filling such a need.
In making their invention, the inventors have recognized that to keep the overall package costs to a minimum, it is desirable to design a coupling structure which is mechanically simple and easy to attach to the housing of the waveguide. As part of their invention, the inventors have developed a structure that may be integrated onto a selected portion of a substrate which carries the electrical signal, and that may be coupled to the waveguide by attaching the selected portion of the substrate to an end of the waveguide. The substrate may comprise a printed circuit board, a multichip substrate, or the like. Constructions according to the present invention may be integrated on the same substrate which carries the chip that generates the electrical signal being coupled to the waveguide. Since constructions according to the present invention may be integrated onto an existing substrate that can be constructed with mature and cost-efficient manufacturing processes, the present invention is relatively inexpensive to practice.
The present invention encompasses coupling structures for coupling an electrical signal on a substrate to a waveguide. The substrate has a substrate layer with a first major surface and a second major surface opposite to the first major surface, and the waveguide has a first end, a second end, and a housing disposed between the first and second ends. The substrate layer may comprise a single layer of dielectric material, or may comprise a plurality of dielectric sub-layers and conductive (e.g., metal) sub-layers interleaved with respect to one another. The waveguide housing defines a longitudinal dimension between the first and second ends along which electromagnetic waves may propagate. The waveguide housing has one or more walls which form a lip at one waveguide end, to which constructions according to the present invention may be attached.
An exemplary structure according to the present invention comprises a ground ring located on the first major surface of the substrate layer and adapted for contact with the lip at an end of a waveguide, a first area enclosed by the ground ring, and a ground plane disposed on the second major surface of the substrate layer and located opposite to at least the first area. The exemplary structure further comprises a patch antenna disposed on the first major surface of the substrate layer or within the substrate layer (as may be the case when the substrate layer comprises sub-layers), and further located within the first area. The electrical signal is coupled to the patch antenna, such as by an electrical trace that is conductively isolated from the ground ring and the ground plane.
In preferred embodiments according to the present invention, the electrical signal is conveyed to the patch antenna by a conductive trace disposed on the second major surface of the substrate layer or within the substrate layer (as may be the case when the substrate layer comprises sub-layers), and a conductive via formed in the substrate layer, and preferably through the substrate layer between the first and second major surfaces. The conductive via is electrically coupled to the patch antenna and to the conductive trace.
Preferred embodiments of the present invention further comprise a capacitive diaphragm disposed on the substrate layer""s first major surface or within the substrate layer (as may be the case when the substrate layer comprises sub-layers), and further located between the patch antenna and the ground ring. The capacitive diaphragm enables a better matching of the impedance of the conductive trace to the impedance of the waveguide, and thus enables the constructions according to the present invention to operate over a wide range of frequency.
Accordingly, it is an object of the present invention to provide coupling structures for coupling an electrical signal on a substrate to a waveguide which are inexpensive to construct.
It is another object of the present invention to provide such coupling structures which are compact in size and which can be easily coupled to a waveguide.
It is yet another object of the present invention to provide such coupling structures which are simple in construction and which can be readily mass produced.
It is still another object of the present invention to provide such a coupling structure which can have its operating frequency set to any value over a wide range of frequencies with the addition of a simple and compact component.
It is a further object of the present invention to minimize the packaging costs of MMICs which have output signals coupled to waveguides and/or input signals which are received from waveguides.
It is yet another object of the present invention to provide a substrate-to-waveguide coupling structure which does not require structural modifications to the waveguide.
These and other objects of the present invention will become apparent to those of ordinary skill in the art upon review of the present Specification and the attached claims.